1. Field of the Invention
The present invention relates to a method of operating a nuclear power plant and the nuclear power plant and a method of controlling water chemistry of the nuclear power plant in which occurrence of stress corrosion cracking in metallic components in contacting with reactor cooling water of the nuclear power plant is suppressed.
2. Prior Art
Presently, hydrogen injection to reactor water is widely applied to a boiling water reactor as one of measures for preventing occurrence of intergranular stress corrosion cracking (hereinafter, referred to as IGSCC) in a metallic component material of a nuclear reactor component (hereinafter, appropriately referred to as nuclear reactor component material) in contact with the reactor water such as a reactor pressure vessel, a nuclear reactor internal components or piping. There is an electrochemical corrosion potential (hereinafter, referred to as ECP) of a metallic component material as a potential of IGSCC, and it is said that the potential of IGSCC increases as the ECP becomes higher when the ECP exceeds a critical value of ECP (in a case of stainless steels used in the most reactor components, the critical value of ECP is about xe2x88x92230 mV vs.SHE). The hydrogen injection has a function to decease the ECP of metallic component materials.
As a technology of effectively lowering the ECP with injecting a small amount of hydrogen, noble metal injection is disclosed in Japanese Patent Application Laid Open No.7-198893, Japanese Patent Publication No.2818943. In this technology, a solution containing a noble metal typical of platinum, rhodium, palladium is injected into reactor water, and hydrogen is also injected into the reactor water. The injected noble metal is deposited in surfaces of the nuclear reactor components such as the reactor pressure vessel and the reactor internal components, and recombination of oxygen and hydrogen is accelerated by catalysis of the noble metal to form water molecules. As the result, an amount of oxygen is decreased, and the ECP of the nuclear reactor internal components can be decreased lower than the critical value of ECP with a small amount of injected hydrogen.
On the other hand, as technologies of lowering the electrochemical corrosion potential of the nuclear reactor component material without injecting hydrogen, non-noble metal injection is disclosed in Japanese Patent Application Laid-Open No.7-311295, Japanese Patent Application Laid-Open No.8-43587, Japanese Patent Application Laid-Open No.10-197684 and Published Japanese Translation of PCT International Publication for Patent Application No.9-502533. The technologies decrease the ECP of nuclear reactor component materials without injecting hydrogen by doping non-noble metallic spicies typical of zirconium into oxide films on surfaces of the nuclear reactor component materials.
Further, Japanese Patent Application Laid-Open No.8-226994 discloses both of technologies that the electrochemical corrosion potential of the nuclear reactor component materials is decreased with a small of injecting hydrogen by injecting a noble metal and hydrogen into reactor water of a nuclear reactor, and that the electrochemical corrosion potential of the nuclear reactor component materials is decreased by injecting a non-noble metal and hydrogen into reactor water of a nuclear reactor. It is said that the latter technology is applicable even to the case without hydrogen injection.
Presently, hydrogen injection to reactor water is widely applied to a boiling water reactor as one of measures for preventing occurrence of IGSCC. However, a large amount of hydrogen is necessary to be injected in order to decrease the ECP down to a value lower than the critical value of ECP. Increase in the amount of hydrogen injection causes increase in an amount of radioactive nitrogen exhausted to the main steam system, which increases the radiation dose rate of the main steam system.
According to the technologies injecting the solution containing noble metal disclosed in Japanese Patent Application Laid Open No.7-198893, Japanese Patent Application Laid-Open No.8-226994 and so on, the ECP of the nuclear reactor component material can be decreased to a value below the critical value of ECP with a smaller amount of hydrogen injection compared to the case without the noble metal injection. However, in the noble metal injection, it is necessary to perform control in taking an effect on corrosion of the cladding tubes when the injected noble metal is attached onto the surfaces of fuel cladding tubes. In addition to this, there is a problem in that the operating cost is increased due to use of the noble metal.
In the technologies injecting the non-noble metal disclosed in Japanese Patent Application Laid Open No.7-311295, Japanese Patent Application Laid-Open No.8-226994 and so on, zirconium, hafnium, tantalum, niobium, yttrium and so on are shown as the non-noble metals, and acetylacetonato zirconium, zirconium nitrate, zirconyl nitrate are shown as the chemical compounds containing zirconium. By injecting these chemical compounds into reactor water, the ECP can be decreased without hydrogen injection. However, the patents do not disclose any sufficient knowledge on decrease of the ECP, and it cannot said, as far as the contents of Detailed Description of the Invention, that the ECP of the treated material is decreased down to the critical value of ECP capable of suppressing IGSCC. Further, there is a possibility that increase in the conductivity of the cooling water of nuclear reactor due to nitrate group at injection treatment may increase the burden of water chemistry control of the plant.
Further, according to a test conducted by the inventors of the present invention, it is found that a phenomenon completely opposite to decrease of the ECP when zirconium hydroxide is used. That is, when zirconium hydroxide is injected into reactor water of the nuclear reactor without hydrogen injection, the ECP is not decreased but inversely increased, which is different from the phenomenon described in the above-mentioned patents.
An object of the present invention is to provide a method of operating a nuclear power plant, the nuclear power plant and a method of controlling water chemistry of the nuclear power plant in which in the nuclear power plant, by injecting an amount of hydrogen small enough not to increase a radiation dose rate of the main steam system, ECP of metallic component materials composing a nuclear reactor can be decreased to suppress the potential of occurrence of IGSCC, and the control can be easily performed, and the operating cost can be suppressed to increase.
(1) In order to attain the above object, in the present invention, a method of operating a nuclear power plant for suppressing occurrence of stress corrosion cracking in metallic component materials in contact with reactor cooling water of a nuclear power plant, wherein an electrochemical corrosion potential of the metallic component material is decreased by injecting zirconium hydroxide and hydrogen into the reactor cooling water.
As described above, according to the result of the test conducted by the inventors of the present invention, it is found that on the contrary, the ECP is increased when only zirconium hydroxide is injected into reactor cooling water (hereinafter, referred to as reactor water). However, according to the result of a further test conducted by the inventors of the present invention, it is found that when both of zirconium hydroxide and hydrogen are injected into reactor water, the ECP is substantially decreased compared to in the case of injecting only hydrogen.
Furthermore, as a result of studying these test results, it is estimated that the decrease in the ECP by injecting zirconium hydroxide is different in principle and mechanism from those in the prior art, that is, the decrease in the ECP in the case of injecting only hydrogen or the noble metal and hydrogen into the reactor water, or the decrease in the ECP in the case of injecting only the non-noble metal into the reactor water.
That is, in the case where only hydrogen is injected into the reactor water, it is said that water molecules are formed by recombination of oxygen and hydrogen, and as the result, the oxygen concentration in the reactor water is decreased to decrease the ECP. Further, in the case where the noble metal and hydrogen are injected into the reactor water, it is stated in the published patents described above that recombination of oxygen and hydrogen is accelerated by the catalysis of the noble metal attached on the surfaces of the reactor components, and as the result, the oxygen concentration in the reactor water is decreased to decrease the ECP with the small injection amount of hydrogen.
On the other hand, in the case of the present invention where zirconium hydroxide and hydrogen are injected, it can be estimated that films containing zirconium oxide are formed on the surfaces of the nuclear reactor components, and electrochemical oxidation of hydrogen of anodic reaction is accelerated on the films containing zirconium oxide, which decreases the ECP determined by the balance between cathodic reaction (electrochemical reduction of oxygen dissolved in the reactor water) and the anodic reaction (electrochemical oxidation between the metallic component material and hydrogen) at the boundary between the surfaces of the nuclear reactor components (to be described later).
Further, it is stated that the conventional technology of injecting the non-noble metal into the reactor water can be used under the condition without hydrogen injection. On the contrary, in the present invention, although the ECP is inversely increased when only zirconium oxide is injected into the reactor water, the electrochemical oxidation of hydrogen on the films containing zirconium oxide is accelerated by injecting hydrogen together with the zirconium into the reactor water to substantially decrease the ECP. Therefore, a primary factor of substantially decreasing the ECP exists in the hydrogen on the films of zirconium-oxide treated materials, and accordingly the hydrogen injection is inevitable for decreasing the ECP. Thus, the technology of the present invention is different from the conventional technology of injecting the non-noble metal into the reactor water in principle.
As described above, according to the present invention, by injecting a amount of hydrogen small enough not to increase the radiation dose rate of the main steam system, the ECP of the metallic component materials composing the nuclear reactor can be decreased and accordingly the potential of IGSCC can be made smaller.
Further, zirconium is an element used for the reactor component materials such as cladding tubes of fuel rods, and accordingly corrosion of the cladding tubes are not accelerated even if the zirconium is attached onto the surfaces of the cladding tubes as far as the concentration of zirconium is not excessively high, and the control of zirconium concentration is easy. Furthermore, since zirconium hydroxide is very economical, the operating cost is hardly increased.
(2) Further, in order to attain the above object, in the present invention, a method of operating a nuclear power plant for suppressing occurrence of stress corrosion cracking in metallic component materials in contact with reactor cooling water of a nuclear power plant, wherein electrochemical corrosion potentials of the metallic component materials are decreased by injecting zirconium hydroxide and hydrogen into the reactor cooling water, and an injecting amount of the hydrogen, an injecting amount of the zirconium hydroxide and injecting timing of the zirconium hydroxide are controlled so that the electrochemical corrosion potentials may be maintained below a target value.
As described above, by controlling the injecting amount of the hydrogen, the injecting amount of the zirconium hydroxide and the injecting timing of the zirconium hydroxide while the electrochemical corrosion potential is being monitored, the ECP can be maintained below the target value for a long term.
(3) In the above items (1) and (2), it is preferable that the zirconium hydroxide is injected so that a concentration of zirconium in the reactor cooling water may be kept within a range of 0.5 to 50 ppb.
It is estimated that a processing time required for making the ECP lower than xe2x88x92230 mV vs. SHE is about 1000 hours by setting the concentration of zirconium in the reactor cooling water above 0.5 ppb, and one cycle of nuclear reactor operation period is about 10,000 hours. Therefore, it can be thought that the 1,000-hour period is a practically maximum allowable processing time.
Wearing of sliding rotator members and effects of the zirconium attaching to the fuel cladding tubes can be minimized by setting the concentration of zirconium in the reactor cooling water below 50 ppb.
(4) In the above items (1) and (2), it is particularly preferable that the zirconium hydroxide is injected so that a concentration of zirconium in the reactor cooling water may be kept within a range of 1 to 6 ppb.
The processing time required for making the ECP lower than xe2x88x92230 mV vs. SHE is shorter than 500 hours by setting the concentration of zirconium in the reactor cooling water above 1 ppb, which is a practically sufficient allowable processing time.
In taking it into consideration that the maximum concentration of iron crud injected for reducing the radiation dose rate in the present actual operation is around 5 ppb, by setting the concentration of zirconium in the reactor cooling water to a value below 6 ppb, it may be possible to prevent ill effect of the zirconium oxide in the same kind of the oxide particles on the plant components.
(5) In the above items (1) and (2), it is preferable that the hydrogen is injected so that the concentration of hydrogen in feed water may be kept within a range of 0.1 to 0.6 ppm.
In taking it into consideration that a practical ECP reducing effect of about xe2x88x92100 mV vs. SHE can be obtained by setting the concentration of hydrogen in feed water above 0.1 ppm, the target ECP can be obtained.
By setting the concentration of hydrogen in feed water below 0.6 ppm, the operation of the nuclear power plant can be performed at a hydrogen concentration lower than or equal to that of the hydrogen injection operated at present.
(6) In the above items (1) and (2), it is particularly preferable that the hydrogen is injected so that a concentration of hydrogen in feed water may be kept within a range of 0.3 to 0.5 ppm.
It has been experimentally verified that the ECP can be made lower than xe2x88x92230 mV vs. SHE by setting the hydrogen concentration in feed water to 0.5 ppm when the zirconium concentration is 5 ppb, and that the ECP can be made lower than xe2x88x92230 mV vs. SHE by setting the hydrogen concentration in feed water to 0.3 ppm when the zirconium concentration is the upper limit of 50 ppb. Therefore, by setting the hydrogen concentration in the feed water to a value within the range of 0.3 to 0.5 ppm, the ECP can be made lower than xe2x88x92230 mV vs. SHE within the range of zirconium concentration of 5 ppb to 50 ppb.
(7) Further, in the above items (1) and (2), it is preferable that the metallic component materials are materials selected from the group consisting of stainless steels, nickel based alloys, cobalt based alloys, titanium based alloys, copper based alloys and ferroalloys, non-ferrous alloys, carbon steels and low alloy steels.
It has been clarified from a test result that in the case where the metallic component materials are stainless steels, there is an effect of reducing the ECP.
In the case where the metallic component materials are nickel based alloys, cobalt based alloys, titanium based alloys, copper based alloys and ferroalloys, non-ferrous alloys, carbon steels or low alloy steels, there is also an effect of reducing the ECP by similar action of the film containing zirconium oxide formed on the material surface.
(8) Further, in the above items (1) and (2), it is preferable that the injection of zirconium hydroxide is performed during operating the nuclear power plant at a full power.
(9) In the above items (1) and (2), the injection of zirconium hydroxide may be performed during shut-down operation of the nuclear power plant.
(10) Further, in the above items (1) and (2), it is preferable that the injection of hydrogen is initiated after completion of the injection of zirconium hydroxide.
(11) Further, in the above items (1) and (2), it is preferable that an electrochemical corrosion potential of the metallic component material is monitored when the hydrogen is injected, and zirconium hydroxide is additionally injected during operating the nuclear power plant at full power based on the monitored result.
(12) Further, in order to attain the above object, in the present invention, the nuclear power plant comprises a means for injecting zirconium hydroxide into reactor cooling water; and a means for injecting hydrogen into the reactor cooling water.
(13) Further, in order to attain the above object, in the present invention, the nuclear power plant comprises a means for injecting zirconium hydroxide into reactor cooling water; a means for injecting hydrogen into the reactor cooling water; and a means for monitoring an electrochemical corrosion potential of a metallic component material.
(14) Further, in order to attain the above object, in the present invention, a method of controlling water chemistry of a nuclear power plant is that in order to suppress occurrence of stress corrosion cracking in metallic component materials in contact with reactor cooling water of a nuclear power plant, zirconium hydroxide is injected into the reactor cooling water before injecting hydrogen into the reactor cooling water.
(15) In the above item (14), it is preferable that the zirconium hydroxide is injected so that a concentration of zirconium in the reactor cooling water may be kept within a range of 0.5 to 50 ppb.